The present invention relates to a phased array antenna used for transmitting/receiving an RF signal such as a microwave to electrically adjust a beam radiation direction by controlling a phase supplied to each radiating element, and a method of manufacturing the antenna.
As a satellite tracking on-vehicle antenna or satellite borne antenna, a phased array antenna having many radiating elements arranged in an array has conventionally been proposed (see Technical Report AP90-75 of the Institute of Electronics, Information and Communication Engineers, and Japanese Patent Laid-Open No. 1-290301).
A phased array antenna of this type has a function of arbitrarily changing the beam direction by electronically changing the phase of a signal supplied to each radiating element.
As a means for changing the feed phase of each radiating element, a phase shifter is generally used.
As the phase shifter, a digital phase shifter (to be simply referred to as a phase shifter hereinafter) made up of a plurality of phase shift circuits having different fixed phase shift amounts is used.
The phase shift circuits are respectively ON/OFF-controlled by 1-bit digital control signals to combine the phase shift amounts of the phase shift circuits, thereby obtaining a feed phase of 0xc2x0 to 360xc2x0 by the whole phase shifter.
A conventional phased array antenna uses many components including semiconductor elements such as PIN diodes and GaAs FETs serving as switching elements in phase shift circuits, and driver circuit components for driving the semiconductor elements.
The phase shifter applies a DC current or DC voltage to these switching elements to turn them on/off, and changes the transmission path length, susceptance, and reflection coefficient to generate a predetermined phase shift amount.
Recently in the field of low earth orbit satellite communications, communications at high data rates are required along with the wide use of the Internet and the spread of multimedia communications, and the gain of the antenna must be increased.
To implement communications at high data rates, the transmission bandwidth must be increased. Because of a shortage of the frequency resource in a low-frequency band, an antenna applicable to an RF band equal to or higher than the Ka band (20 GHz or higher) must be implemented.
More specifically, an antenna for a low earth orbit satellite tracking terminal (terrestrial station) must satisfy technical performance:
Frequency: 30 GHz
Antenna gain: 36 dBi
Beam scanning range: beam tilt angle of 50xc2x0 from front direction
To realize this by a phased array antenna, first,
the aperture area: about 0.13 m2 (360 mmxc3x97360 mm) is needed.
In addition, to suppress the side lobe, radiating elements must be arranged at an interval of about xc2xd wavelength (around 5 mm for 30 GHz) to avoid generation of the grating lobe.
To set a small beam scanning step and minimize the side lobe degradation caused by the quantization error of the digital phase shifter, the phase shift circuit used for the phase shifter is desirably made up of at least 4 bits (22.5xc2x0 for the minimum-bit phase shifter).
The total number of radiating elements and the number of phase shift circuit bits used for a phased array antenna which satisfies the above conditions are given by
Number of elements for the phase shift circuit:
72xc3x9772=about 5,000
Number of phase shift circuit bits:
72xc3x9772xc3x974=about 20,000 bits
When a high-gain phased array antenna applicable to an RF band is to be implemented by, e.g., a phased array antenna disclosed in Japanese Patent Laid-Open No. 1-290301 shown in FIG. 19, the following problems occur.
More specifically, a conventional phased array antenna controls phase shift circuits in each phase shifter by one driver circuit, as shown in FIG. 19. For this purpose, the driver circuit must be connected to all the phase shift circuits.
This requires connection wiring lines equal in number to the number of radiating elementsxc3x97the number of phase shift circuit bits. If the above numerical values are applied, the number of wiring lines to phase shift circuits (4 bits) for one line (72 radiating elements) is 72xc3x974=288 in an array of 72xc3x9772 radiating elements.
If these wiring lines are formed on a single plane, the width of a wiring line bundle for one line (72 radiating elements) is 0.1 mmxc3x97288=28.8 mm for the wiring line width/wiring line interval (L/S)=50/50 xcexcm.
To the contrary, in a phased array antenna applicable to a frequency of 30 GHz, radiating elements must be arranged at an interval of around 5 mm, as described above. In the prior art, however, radiating elements cannot be physically arranged because the width of the wiring line bundle is large.
Accordingly, such a prior art implements no high-gain phased array antenna applicable to an RF band.
If, as the prior art, discrete components which construct the phase shifter, e.g., switching elements and its driver circuits are individually mounted on the substrate, the number of mounting components greatly increases in accordance with an increase in number of radiating elements.
This increases a time required for mounting these components on the substrate and the manufacturing lead time, thereby increasing manufacturing cost.
The present invention has been made to solve the above problems, and has as its object to provide a high-gain phased array antenna applicable to an RF band.
To achieve the above object, in a phased array antenna according to the present invention, radiating elements and phase shift units are individually formed on a radiating element layer and phase control layer, respectively, and both layers are coupled by a first coupling layer to form a multilayered structure as a whole. A distribution/synthesis unit is formed on a distribution/synthesis layer, and the phase control layer and distribution/synthesis layer are coupled by a second coupling layer to form the multilayered structure as a whole. Therefore, the radiating elements and distribution/synthesis unit are eliminated from the phase control layer, thereby reducing an area in the phase control layer which is to be occupied by the radiating element and distribution/synthesis unit.
The phase shift units are connected in a matrix by signal lines and scanning lines, and the signal lines and the scanning lines are matrix-driven to set desired phase shift amounts to phase shift units located at intersections between the signal lines and the scanning lines. The signal wiring lines for controlling the phase shift units can be shared to greatly reduce the number of wiring lines.
A driver circuit constructing the phase shift unit is formed from a thin-film transistor on a glass substrate, and a micromachine switch is used in a phase shifter. This can reduce an area which is to be occupied by these circuit components in comparison with the prior art.
Accordingly, since one phase shift unit is formed in a very small area, many radiating elements are arranged, in units of several thousands, at an interval (around 5 mm) which is optimal for an RF signal of, e.g., about 30 GHz. This can implement a high-gain phased array antenna applicable to an RF band.
In addition, switching elements and circuit components used in each phase shift unit are simultaneously formed on a phase control layer (a single substrate). Therefore, as compared to a case wherein the circuit components are individually mounted as in the prior art, the numbers of mounting components, the numbers of connections, and the numbers of assembling processes can decrease, thereby reducing the manufacturing cost of the whole phased array antenna.